基础化学_10 solutions

1 Chemistry for engineering solutions Silvia Bodoardo 1 2 Chemistry for engineering solutions Silvia Bodoardo 2 • definition • solubility • concentration •colligative properties • elettrolites colligative properties CONTENTSCONTENTS 3 Chemistry for engineering solutions Silvia Bodoardo 3 SOLUTIONSSOLUTIONS A solution is an homogenous mixture of atoms, molecules or ions distributed in the whole space. The most abundant component is the solvent: solutes dissolve in solvents Gas mixtures liquid solutions solid solutions 4 Chemistry for engineering solutions Silvia Bodoardo 4 SOLUBILITYSOLUBILITY similia similibus solvuntur When the bonds in the solute and in the solvent are of the same nature, solubility is possible when excess undissolved solute is in dinamic equilibrium with the dissolved solute, the solution is called saturated. Solubility depens on Kps (v.) 5 Chemistry for engineering solutions Silvia Bodoardo 5 + + + + + +++ - -- -- - - SOLVATAZIONESOLVATAZIONE + - For ionic compounds, when ions are surronded by solvent molecules adequately oriented, ions become solvated. 6 Chemistry for engineering solutions Silvia Bodoardo 6 metanole solubility in water 7 Chemistry for engineering solutions Silvia Bodoardo 7 DISSOLUTION OF SOLUTESDISSOLUTION OF SOLUTES For ionic compounds, when ions are surronded by solvent molecules adequately oriented, ions become solvated.. 8 Chemistry for engineering solutions Silvia Bodoardo 8 ExamplesExamples of of solventsolvent--solutessolutes interactioninteraction 9 Chemistry for engineering solutions Silvia Bodoardo 9 quantitative quantitative waysways of of expressingexpressing the the concentrationconcentration of a of a solutionsolution mass percent: g solute in 100 g solution volume percent: ml solute in 100 ml solution MOLARITY (M): solute moles in 1 liter solution MOLALITY (m): solute moles in 1000 g solvent mole fraction: χs = n/(n+N) ; n = solute moles; N = solvent moles 10 Chemistry for engineering solutions Silvia Bodoardo 10 ColligativeColligative propertiesproperties Some properties of the solution depend on the amount of the solute and of the solvent and are indipendent by the type of solute. These properties are called colligative properties. The most important colligative properties are: • vapour presure lowering • boiling point elevation • freezing point depression • osmotic pressure Laws regulating colligative properties are different for electrolyte and non electrolyte 11 Chemistry for engineering solutions Silvia Bodoardo 11 VapourVapour pressurepressure of of solutionssolutions Raoult law (solid solute, liquid solvent): (p° - P)/p° = n/n+N The vapour pressure of a solution, in the same experimental conditions, are always lower than the one of the pure solvent. P: total vap. pres. p°:solvent vap. pres. n: solute moles N: solvent moles According to Raoult, the vapour pressure of a solvent is proportional to its mole fraction in the solution It can be used only for ideal solutions (Raoult solutions) 12 Chemistry for engineering solutions Silvia Bodoardo 12 Ideal solution: the solution is diluited The vapor pressure of the solute is negligible respect to that of the solvent the solute does not bond to the solute and does not dissociate The solute does not react with the solvent 13 Chemistry for engineering solutions Silvia Bodoardo 13 14 Chemistry for engineering solutions Silvia Bodoardo 14 BoilingBoiling pointpoint elevationelevation: ∆Teb = keb m FreezingFreezing pointpoint depressiondepression : ∆Tcr = kcr m ∆Teb = (p°/k). m ; keb = (p°/k) ∆Teb = keb m Characteristic of the sole solvent 15 Chemistry for engineering solutions Silvia Bodoardo 15 osmoticosmotic pressurepressure π V = n RT The osmotic pressure depends on the Volume and the absolute Temperature, according to Van’t Hoff equation 16 Chemistry for engineering solutions Silvia Bodoardo 16 ELETTROLYTES SOLUTIONSELETTROLYTES SOLUTIONS Some substances (salts, acides, bases) dissociates in solution producing ions. Examples: KCl K+(aq) + Cl-(aq) H2SO4 2H+(aq) + SO4--(aq) KOH K+(aq) + OH-(aq) Elettrolytic solutions They are called electrolytes (strong complete dissociation) (weak partial dissociation) 17 Chemistry for engineering solutions Silvia Bodoardo 17 The amount of the dissocation is measured by the dissociation degree α: Dissociation degree α = Ndiss/N Dissociated moles Ndiss = α N Undissociated moles Nind = N - α N Produced ions z α N particles in solution N - α N + z α N = N (1 - α (z - 1)) i - 1α = ------ z - 1 18 Chemistry for engineering solutions Silvia Bodoardo 18 For electrolytes solutions, it is sufficient to add the Van’t Hoff coefficient i in the described expressions : i = 1 + α (z-1) α = dissociation degree z = number of produced ions ™ P = p° - p°(n i/ (n i + N)) ™ ∆Teb = keb m i ™ ∆Tcr = kcr m i ™ π V = n i RT